The present invention relates to the rotor of a superconducting generator and, more particularly, to the construction and method for making a warm damper for a supercooled rotor.
Superconducting generators typically utilize supercooled rotor field windings which conduct a direct current through its coils. This direct current experiences essentially no resistance due to its supercooled state and significant reductions of power loss can thus be achieved.
In order to maintain stability, however, amplitude variations in this direct current must be avoided. Possible causes of these variations include transient faults which can subject the rotor windings to time-varying magnetic fields.
The coils of a superconducting rotor's winding can be shielded from these time-varying magnetic fields by interposing a damper between them and the source of these magnetic fields, namely the generator's stator winding. This damper, referred to herein as a warm damper because it is not itself supercooled, is a tubularly shaped construction disposed around the rotor's winding. It comprises a conductive cylinder which is capable of absorbing externally caused magnetic fields. Although pure copper does not have the required mechanical strength for this application, this conductive central cylinder can be made of a higher strength zirconium-copper alloy although other suitable high strength and conductivity materials, such as chromium-copper, can be used as a viable substitute. The magnetic field is absorbed by the central cylinder as the field's strength is dissipated by the creation of eddy currents within the central cylinder.
This magnetic field dissipation produces potentially significant forces within the central shield which could possibly cause severe distortions within it. Therefore, some means for rigidly supporting the central cylinder is required. A high strength superalloy material can be used to provide support on the central cylinder's radially inside and outside surfaces. Superalloys are iron, cobalt or nickel based alloys that contain chromiumm for resistance to oxidation and hot corrosion and that contain other elements for strength at elevated temperatures. It should be understood that any material which has a yield strength in excess of 130,000 psi and has sufficient elongation properties to be suitable for use in a superconducting rotor application, such as Inconel 706 or Inconel 718, is within the scope of the present invention and can be used as an alternative for the superalloy discussed herein. This construction, effectively, a superalloy-copper-superalloy laminar assembly and is capable of providing sufficient mechanical strength for the central cylinder under supercooled conditions. However, it presents manufacturing problems if the laminar assembly is to be attached with other components located at its axial ends.
The construction described above results in the assembly having axial surfaces which comprise three concentric rings of dissimilar materials, namely Inconel-copper-Inconel in the above example. These surfaces are not suitable for maintaining a bolted connection to other components since the central cylinder of copper does not have the mechanical strength to withstand the forces associated with a generator rotor. In contradistinction, axial surfaces which are totally constructed of an Inconel alloy would provide sufficient strength to maintain a bolted connection to another rotor component which is disposed axially adjacent to the above described assembly.
The present invention provides a composite assembly which has a central cylinder of a high strength copper alloy which is completely encased in shell made of a high strength material capable of retaining its properties at cryogenic temperatures or any suitable superalloy, resulting in axial surfaces which have no copper portion. A warm damper made in accordance with the present invention comprises a central conductive cylinder which has a high strength cylinder made from a superalloy and bonded, by soldering or brazing, to each of its ends in an aligned coaxial relation. These three cylinders each have a central bore therethrough and each have essentially identical inside and outside diametric dimensions. This composite assembly is then disposed between an inner and an outer high strength superalloy cylinder in concentric and coaxial relation. The composite cylinder is then bonded to the inner and outer cylinder along their adjacent circumferential surfaces. The composite, inner and outer cylinders are bonded together by explosive welding.
The combination of soldering, or brazing, and explosive welding provides a sufficient metallurgical bonding between all adjacent surfaces of the warm damper. This bonding has the required strength to withstand the stresses associated with a generator rotor and permits the warm damper to be constructed in a way that results in its cylindrical shape having two axial surfaces which can readily be bolted to other rotor components.